This invention relates to a blast plug and stemming construction for explosive loaded blast holes.
Blasting is used in construction and mining to fragment solid rock so that it can be removed. A number of blast holes are drilled and filled with explosive charges which are detonated to produce shock waves that rupture the surrounding rock. There are a number of parameters that govern the effectiveness of a blast, including geologic structure, the size and spacing of the blast holes, the burden (distance to the free face), the type, amount, and placement of explosive, the sequence of detonation, and the stemming technique used.
Stemming is the plugging of the blast hole to prevent the escape of blast gasses. This is important because the blast gasses perform the primary work of the blast. If the blast gasses escape, the effectiveness of the blast is diminished, wasting explosive and requiring additional blasting which entails additional risk and increased drilling, labor, and material costs. Stemming is also important because escaping gasses create an overpressure or air concussion causing objectionable noise and possibly causing personal injury or property damage.
U.S. Pat. No. 4,754,705 to Worsey, incorporated herein by reference, describes a steaming construction having a tapering wedge-shaped blast plug. The plug is positioned in the blast hole outwardly of an explosive charge with its narrower end facing outwardly toward the mouth of the blast hole. Upon detonation of the explosive charge, the plug is driven outward into stemming material in the blast hole to wedge the stemming material laterally against the walls of the blast hole. A stabilizing rod extends endwise from the narrow end of the plug and resists forces tending to tilt the plug relative to the blast hole. Optionally, the rod may be provided with discs to help center the rod and the plug in the blast hole.
Increasing the length of the aforementioned stabilizing rod increases the stability of the plug, i.e., decreases the tendency of the plug to tilt relative to the blast hole. Generally, good stability requires a long rod. However, the longer the rod, the more expensive it is to manufacture and ship.
Often, the rod is used to orient the plug in the blast hole but removed when the plug is in position. Since, in this case, the rod is not fixed to the plug, the plug may come loose from the rod during insertion and may not be properly oriented (e.g., the plug may be tilted relative to the blast hole). Even if the plug is properly inserted, without a stabilizing device it may tilt due to slumping of powder and stemming material in the hole. Improper orientation of the plug within the blast hole reduces the effectiveness of the stemming.